Sits inside your aquarium or pond filter. 10 Bio-balls. All Natural Biological filtration for Nitrifying bacteria. They can be used in freshwater and saltwater filters, as the water runs over the balls the nitrifying bacteria filters your water removing ammonia and nitrites which improves the water quality. Clean them by simply wiping them with your aquarium water, (not tap water) and they last forever. Can be used in conjunction with other filter media.